[zk-token-sdk] clean-up zk-token-elgamal submodule in zk-token-sdk

[samkim-crypto]

Problem

The zk-token-elgamal submodule contains pod types for various types in zk-token-sdk. Currently, the code in this submodule is quite messy, docs are missing, and overall quite hard to read. Cleaning up zk-token-elgamal is long overdue.

Proposed Solution

Clean up zk-token-elgamal in a series of steps:

• refactor the pod types that live in zk-token-elgamal/pod into separate submodules
• refactor the conversion logic in convert into separate submodules that were created in the previous step
• refactor decrypt.rs and ops.rs into a single submodule
• clean-up and add documentation